1. Field
The present invention relates to prepregs, towpregs and preforms made from curable compositions of (a) benzoxazines, (b) a combination of adducts a first of which is prepared from hydroxy-containing compounds, isocyanate-containing compounds and phenolic compounds and the second of which is prepared from the first adduct and epoxy-containing compounds, (c) components reactive with the benzoxazine, such as epoxy resins, episulfide resin and oxetane resins, (d) tougheners, such as polyether sulfones, and (e) a silica component, where the silica has a mean particle diameter on the order of 10−9 meters.
2. Brief Description of Related Technology
Epoxy resins with various hardeners have been used extensively in the aerospace industry both as adhesives and as matrix resins for use in prepreg assembly with a variety of substrates.
Mixtures of epoxy resins and phenol-capped polyurethanes are known. Polyurethanes are obtained ordinarily by reacting isocyanates with hydroxy-containing compounds; the resulting polyurethane products should no longer contain free, phenolic hydroxyl groups. Such polyurethane products may be combined with epoxy resins and amine curing agents to give curable coating agents reportedly distinguished by improved elasticity. For instance, see U.S. Pat. Nos. 4,423,201 and 3,442,974.
Epoxy resins can also be mixed with copolymers based on butadiene and acrylonitrile to enhance the impact strength and/or the flexibility of the cured product. Ordinarily, however, such copolymers ordinarily compromise the tensile shear strength and the glass transition temperature of the resulting cured products.
U.S. Pat. No. 5,278,257 (Mulhaupt) refers to and claims a composition containing a copolymer based on at least one 1,3-diene and at least one polar, ethylenically unsaturated comonomer, a phenol-terminated polyurethane, polyurea or polyurea-urethane of a certain formula, after the removal of the terminal isocyanate, amino or hydroxyl groups, which is soluble or dispersible in epoxy resins, it being necessary for at least one of these groups to be a tertiary amine and where the ratio by weight of the comonomer to the polyurethane, polyurea or polyurea-urethane is from 5:1 to 1:5, and an epoxy resin having at least two 1,2-epoxide groups per molecule.
U.S. Patent Application Publication No. 2005/0070634 describes a composition comprising a) one or more epoxy resins; b) one or more rubber modified epoxy resins; c) one or more toughening compositions comprising the reaction product of one or more isocyanate terminated prepolymers and one or more capping compounds having one or more bisphenolic, phenolic, benzyl alcohol, aminophenyl or, benzylamino moieties where the reaction product is terminated with the capping compound; d) one or more curing agents and one or more catalysts for epoxy resins which initiates cure at a temperature of about 100° C. or greater; and e) optionally, fillers, adhesion promoters, wetting agents and rheological additives useful in epoxy adhesive compositions. The resulting adhesive composition is reported to have a viscosity at 45° C. of about 20 Pa·s to about 400 Pa·s.
Benzoxazines have been reported in the literature as generally electrical properties (e.g., dielectric constant), and low flammability.
U.S. Pat. No. 6,323,270 (Ishida) speaks to and claims a nanocomposite composition of clay and benzoxazine monomer, oligomer, and/or polymer in amount effective to form a nanocomposite. The clay is described as a silicate comprised of multiple platelets, or a hydrated aluminum silicate comprised of multiple platelets. Clay, reads the '270 patent, is typically derived from the weathering of rocks that can be an aggregate having a particle size of less than about 200 microns, such as less than about 100 microns, like less than about 50 microns, examples of which being montmorillonite, atapulgite, illite, bentonite, and halloysite.
Blends of epoxy resins and benzoxazines are also known. See e.g. U.S. Pat. Nos. 4,607,091 (Schreiber), 5,021,484 (Schreiber), 5,200,452 (Schreiber), and 5,445,911 (Schreiber). These blends appear to be potentially useful commercially, as the epoxy resins can reduce the melt viscosity of benzoxazines allowing for the use of higher filler loading while maintaining a processable viscosity. However, epoxy resins oftentimes undesirably increase the temperature at which benzoxazines polymerize.
Ternary blends of epoxy resins, benzoxazine and phenolic resins are known as well. See U.S. Pat. No. 6,207,786 (Ishida), and S. Rimdusit and H. Ishida, “Development of new class of electronic packaging materials based on ternary system of benzoxazine, epoxy, and phenolic resin”, Polymer, 41, 7941-49 (2000).
Blends of benzoxazines and curable materials other than epoxy and/or phenolics are also known. To that end, U.S. Pat. No. 6,620,925 (Musa) is directed to and claims a curable composition comprising certain benzoxazine compounds without reactive functionality other than the benzoxazine (apart from allyl and propargyl which are disclosed but not claimed) and a curable compound or resin selected from vinyl ethers, vinyl silanes, compounds or resins containing vinyl or allyl functionality, thiol-enes, compounds or resins containing cinnamyl or styrenic functionality, fumarates, maleates, acrylates, maleimides, cyanate esters, and hybrid resins containing contain both vinyl silane and cinnamyl, styrenic, acrylate or maleimide functionality.
In addition, U.S. Pat. No. 6,743,852 (Dershem) discloses combinations of liquid benzoxazines and a thermosetting resin composition for adhering materials with dissimilar coefficients of thermal expansion comprising a) a benzoxazine compound in liquid form, b) thermoset compounds including epoxy, cyanate ester, maleimide, acrylate, methacrylate, vinyl ether, styrenic, vinyl ester, propargyl ether, diallyl amide, aromatic acetylene, benzocyclobutene, thiolenes, maleate, oxazoline, and itaconate, c) optionally, one or more anti-oxidants, bleed control agents, fillers, diluents, coupling agents, adhesion promoters, flexibilizers, dyes and pigments, and d) a cure initiator.
Y. Cui, et al. have reported on the synthesis and characterization of polyurethane/polybenzoxazine-based interpenetrating polymer networks. See Y. Cui, et al., “Synthesis and Characterization of Polyurethane/Polybenzoxazine-Based Interpenetrating Polymer Networks (IPNs)”, School of Chemistry and Chemical Technology, Polymer International, 52:1246-1248 (2003).
T. Takeichi, et al. have reported on the synthesis and characterization of poly(urethane-benzoxazine) films as novel type of polyurethane/phenolic resin composites. See T. Takeichi, et al., “Synthesis and Characterization of Poly(urethane-benzoxazine) Films as Novel Type of Polyurethane/Phenolic Resin Composites”, School of Material Science, Toyohashi University of Technology, 4165-4176 (2000).
To date however it is believed that curable composition based on the combination of benzoxazines, adducts of epoxy resin, hydroxy (such as phenolic) end-capped polyurethane compounds and phenolic compounds, epoxy resins, polyether sulfone tougheners and silica having a mean particle diameter on the order of 10−9 meters have not been sold or reported. Nor have prepregs, towrpregs or performs made from such curable compositions been sold or reported either.